how do I bevel the control point of a curve?
You could scale curve point to its hadles' plane.

First, for better results make the type of handles of the control points to Free (with all curve points selected press V > Free).


Select the handles of the curve vertice in the corner (where you'd like to make 'bevel'). Hold Shift to select both them.
Press Shift+S to snap the cursor to selection.
Select the curve vertice itself, and deselect the handles (click on them with MMB).
Press S to scale the control point to the cursor location:




Note that it's also possible to change the location of the handles in order to tweak the cursor location.